In this type of technical field, next-generation communication schemes, which may become successors of W-CDMA (Wideband Code Division Multiple Access) or HSDPA (High Speed Downlink Packet Access) systems, are being discussed by W-CDMA standardization group called 3GPP (The 3rd Generation Partnership Project). A typical one of the next-generation communication schemes is an LTE (Long Term Evolution) system. As radio access schemes in the LTE system, an OFDM (Orthogonal Frequency Division Multiple Access) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme are used for downlink and uplink, respectively. See non-patent documents 1 and 2, for example. For convenience, the LTE system is illustratively described below, but the present invention is not limited to the specific system.
The OFDM scheme is a multi-carrier based technique for transmitting data in several narrower frequency bands (subcarriers) resulting from segmentation of a frequency band. By arranging the subcarriers on the frequency densely without mutual interference while partially overlapping the subcarriers, it is possible to achieve fast transmissions and improve the frequency utilization efficiency.
The SC-FDMA scheme is a single-carrier based technique where Fourier transform and inverse Fourier transform are applied to segment a frequency band, so that several terminals can use the segmented different frequency bands. The SC-FDMA scheme has characteristics of reduction in inter-terminal interference, smaller variations of transmission power, and so on. As a result, it is advantageous to reduction in power consumption, wider coverage, and so on.
In LTE system, one or more physical channels are shared in both uplink and downlink communications among several mobile stations (user equipment terminals). The channel shared among the several mobile stations is generally referred to as a shared channel. In the LTE system, the shared channel is referred to as a PUSCH (Physical Uplink Shared Channel) for uplink and a PDSCH (Physical Downlink Shared Channel) for downlink. Also, transport channels mapped into the PUSCH and the PDSCH are referred to as an UL-SCH (Uplink-Shared Channel) and a DL-SCH (Downlink-Shared Channel), respectively.
Also, it is necessary in a communication system using these shared channels to signal which of the shared channels is to be assigned to which of mobile stations for each subframe. In the LTE system, the control channel for the signaling is referred to as a PDCCH (Physical Downlink Control Channel). The PDCCH is also referred to as a downlink L1/L2 control channel, a DL L1/L2 control channel, or downlink control information (DCI). The PDCCH includes information items such as a DL/UL scheduling grant and a TPC (Transmission Power Control) bit. See non-patent document 3.
More specifically, the DL scheduling grant may include downlink resource block assignment information, an ID of a user equipment (UE) terminal, the number of streams, information about precoding vectors, information about a data size and a modulation scheme, information about HARQ (Hybrid Automatic Repeat reQuest), and so on. The DL scheduling grant may be referred to as DL assignment information, DL scheduling information, and so on, for example.
Also, the UL scheduling grant may include uplink resource block assignment information, an ID of a user equipment (UE) terminal, information about a data size and a modulation scheme, uplink transmission power information, information about a demodulation reference signal, and so on, for example.
The PDCCH is mapped into the first one to three OFDM symbols in fourteen OFDM symbols within one subframe, for example. It is specified how many leading OFDM symbols the PDCCH are to be mapped into through a PCFICH described below, which is indicated to a user equipment terminal.
Also, the PCFICH (Physical Control Format Indicator Channel) and a PHICH (Physical Hybrid ARQ Indicator Channel) are transmitted in the OFDM symbols including the PDCCH.
The PCFICH is a signal indicating the number of OFDM symbols (including the PDCCH) to a user equipment terminal. The PCFICH may be referred to as a DL L1/L2 control format indicator.
The PHICH is a channel for transmitting acknowledgement information for the PUSCH. The acknowledgment information includes ACK (Acknowledgement) as a positive response and NACK (Negative Acknowledgement) as a negative response. In other words, the PHICH is a control channel for transmitting a control signal indicating whether the PUSCH is to be retransmitted according to HARQ control.
The HARQ control for the PUSCH and the acknowledgement information transmitted via the PHICH are described below. See non-patent document 4.
In the PUSCH in the LTE system, a synchronous HARQ scheme is used as one of HARQ schemes.
Specifically, as illustrated in FIG. 1, uplink shared signals are transmitted via the uplink shared channel at predetermined timings, more specifically at predetermined intervals, beginning at the first transmission timing.
In FIG. 1, uplink shared signals are retransmitted at the intervals of eight subframes. It should be noted that the uplink shared signals may be retransmitted at different intervals.
The base station apparatus instructs, via an HARQ indicator channel or an UL scheduling grant, a user equipment terminal to retransmit an uplink shared signal.
When the retransmission of the uplink shared signal is instructed via the HARQ indicator channel, in other words, when information identified in the HARQ indicator channel is NACK, the user equipment terminal retransmits the uplink shared signal using the same resource block and the same modulation scheme as those used in the previous transmission.
On the other hand, when the retransmission of the uplink shared signal is instructed via the UL scheduling grant, the user equipment terminal retransmits the uplink shared signal using a resource block and a modulation scheme as specified by the UL scheduling grant.
When the user equipment terminal receives both the HARQ indicator channel and the UL scheduling grant in a certain subframe, the user equipment terminal follows the UL scheduling grant. In this case, information identified in the HARQ indicator channel is discarded.
Referring to FIG. 2, HARQ control for uplink in the LTE system is described in detail below. FIG. 2 illustrates an example of HARQ control for uplink.
As illustrated in FIG. 2, at 202 (in a subframe #i where i is a positive integer), using an uplink scheduling grant in the physical downlink control channel, the base station apparatus instructs the user equipment terminal to communicate using the uplink shared channel in a subframe #i+4.
At 204 (in the subframe #i+4), the user equipment terminal transmits an uplink shared signal to the base station apparatus. The base station apparatus receives and attempts to decode the uplink shared signal.
At 206 (in a subframe #i+8), the base station apparatus transmits an HARQ indicator channel or an UL scheduling grant based on the decoding result.
More specifically, when the decoding result of the uplink shared signal is OK, the base station apparatus transmits an HARQ indicator channel indicating ACK.
Alternatively, when new data to be transmitted are stored in the transmission buffer of the user equipment terminal, the base station apparatus may transmit a new uplink scheduling grant instructing the user equipment terminal to transmit an uplink shared signal.
According to the HARQ control for uplink in the LTE system, the ACK may not necessarily mean that the uplink shared signal has been successfully received. Rather, the ACK may be interpreted that the retransmission of the uplink shared signal may be reserved at the immediately subsequent retransmission timing. Thus, when a user equipment terminal receives the ACK, the user equipment terminal reserves the retransmission of the uplink shared signal. Even if the user equipment terminal receives the ACK, the user equipment terminal does not delete the uplink shared signal from the transmission buffer. When the user equipment terminal is instructed via an UL scheduling grant to retransmit the uplink shared signal at the subsequent HARQ retransmission timing, the user equipment terminal retransmits the uplink shared signal. If the number of retransmissions of the uplink shared signal exceeds the maximum number of retransmissions or if the user equipment terminal is instructed to transmit a new uplink shared signal in the HARQ process of the uplink shared signal, the user equipment terminal deletes the uplink shared signal from the transmission buffer.
When the decoding result of the uplink shared signal is NG, the base station apparatus transmits either an HARQ indicator channel indicating NACK or an UL scheduling grant instructing the user equipment terminal to retransmit the uplink shared signal.
At 206 (in the subframe #i+8), when NACK is transmitted via the HARQ indicator channel or when an uplink scheduling grant instructing the user equipment terminal to retransmit the uplink shared signal is transmitted, the user equipment terminal retransmit the uplink shared signal at 208 (in a subframe #i+12).
At 206 (in the subframe #i+8), when ACK is transmitted via the HARQ indicator channel or when an uplink scheduling grant instructing the user equipment terminal to transmit a new uplink shared signal is transmitted, the uplink shared signal which has been transmitted at 204 is not be retransmitted in the subframe #i+12.
[Non-patent document 1] 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006
[Non-patent document 2] 3GPP TS 36.211 (V8.1.0), “Physical Channels and Modulation”, November 2007
[Non-patent document 3] 3GPP TS 36.300 (V8.2.0), “E-UTRA and E-UTRAN Overall description”, September 2007
[Non-patent document 4] 3GPP TS 36.321 (V8.1.0), “E-UTRA Medium Access Control (MAC) protocol specification”, March 2008